1. Field of the Invention
The invention relates to a dielectric ceramic, a method for manufacturing the dielectric ceramic, and a laminated ceramic capacitor composed with use of the dielectric ceramic, and in particular, relates to an improvement for enhancing the reliability of the laminated ceramic capacitor.
2. Description of the Related Art
While a variety of rated voltages are used for laminated ceramic capacitors, high insulation properties (insulation resistance) and reliability (lifetime characteristics in a high temperature load test) are required for the ceramic material constituting dielectric ceramic layers of, in particular, laminated ceramic capacitors to which middle to high voltages (several tens V to several hundreds V) are applied.
In addition, in the past several years, the need for reduction in size of laminated ceramic capacitors has increased. Thus, in order to ensure at least a predetermined capacitance in the case of small sizes, thinner dielectric ceramic layers have been demanded. However, the thinner the dielectric ceramic layers are, the higher is the intensity of an electric field applied to each of the dielectric ceramic layers. The higher intensity effects achieving the reliability, in particular, higher lifetime characteristic in a load test, which has been required for the dielectric ceramic.
Dielectric ceramics which can satisfy such demands are disclosed, for example, in International Publication No. WO2004/067473.
International Publication No. WO2004/067473 discloses a dielectric ceramic containing: a main component composed of barium titanate based composite oxide with some Ba substituted with Gd and some Ti substituted with Mg, which is represented by the general formula: (Ba1-h-i-mCahSriGdm)k (Ti1-y-j-nZryHfjMgn) O3 and satisfies respective relationships of 0.995≦k≦1.015, 0≦h≦0.03, 0≦i≦0.03, 0.015≦m≦0.035, 0≦y<0.05, 0≦j<0.05, 0≦y+j)<0.05, and 0.015≦n≦0.035; and addition components of Ma (Ma is at least one of Ba, Sr, and Ca) at less than 1.5 moles, however, excluding 0 moles, with respect to 100 moles of the main component, Mb (Mb is at least one of Mn and Ni) at less than 1.0 mole, however, excluding 0 moles, with respect to 100 moles of the main component, and Mc (Mc is Si, or both Si and Ti) at 0.5 moles or more and 2.0 moles or less with respect to 100 moles of the main component.
The main component of the dielectric ceramic described above may not contain Ca, Sr, Zr, or Hf. Thus, one characteristic is that Gd and Mg are present. As described above, Gd is contained at 3.5 mol % or less, whereas Mg is contained at 3.5 mol % or less.
The rare earth element Gd is brought into the A sites of a perovskite structure represented by ABO3 as a solid solution, whereas Mg is brought into B sites of the perovskite structure, thereby achieving high reliability. However, there is a problem in that the substitution contents of Gd and Mg are low and both are 3.5 mol % or less, thereby resulting in the inability to achieve sufficient reliability.
On the other hand, International Publication No. WO2004/067473 discloses a grain diameter of preferably 2.5 μm or less, more preferably 1.5 μm or less, and even more preferably 1 μm or less, for the grain diameters of crystal grains in the dielectric ceramic as a sintered body. In all cases, the crystal grains in the sintered body for the dielectric ceramic described in International Publication No. WO2004/067473, have a large grain diameter of 0.9 μm or more. Accordingly, as thinning is progressed, the number of crystal grains present in each dielectric ceramic layer will be reduced, easily leading to a problem with reliability.
In addition, while a method of increasing the substitution amounts of the rare earth element such as Gd and the element such as Mg is conceivable in order to improve reliability, the crystal grains will likely be further increased in this case.